A typical plant for production of synthetic hydrocarbons by Fischer-Tropsch synthesis such as diesel consists of the following main process units: (a) air separation, (b) synthesis gas preparation via ATR, (c) Fischer-Tropsch synthesis of a raw product of hydrocarbons such as wax and liquid, (d) upgrading comprising hydrocracking and often other refinery steps. More specifically, conventional plants include a reforming section for producing synthesis gas, a down-stream Fischer-Tropsch (FT) synthesis section and an upgrading section. In the reforming section hydrocarbon feedstock, normally natural gas is normally pre-reformed, mixed with part of the off gas (tail gas) from the downstream FT synthesis section, and then passed through an autothermal reformer (ATR) to produce a synthesis gas. An oxygen containing stream is also added to the ATR. The synthesis gas is cooled, condensate is removed and the thus dehydrated synthesis gas is converted to synthetic hydrocarbons via Fischer-Tropsch synthesis. In the FT-synthesis section hydrogen and carbon monoxide react to produce a range of hydrocarbons (including methane, light and heavier paraffins and olefins) and water as well as various by-products for example in the form of oxygenates. An off-gas is also produced.
This off-gas from FT-synthesis is often in the form of a so-called tail gas comprising unreacted hydrogen and carbon monoxide and light hydrocarbons (typically with five or less carbon atoms) including olefins. The tail gas comprises often also carbon dioxide and other typically inert compounds such as nitrogen and argon. The synthetic hydrocarbons may be further upgraded typically resulting in end products such as diesel, naphtha, and LPG. LPG is a mixture of hydrocarbons comprising predominately propane and butane(s).
The above design is typical for a plant in which the main required product is diesel and in which the hydrocarbon synthesis is performed using a so-called low temperature Fischer-Tropsch synthesis with a cobalt based catalyst. However, a similar design may in some cases be used when other synthetic hydrocarbons are the main end product and/or if other types of FT-synthesis catalysts or technologies are employed.
An alternative to autothermal reforming is to produce the synthesis gas by steam reforming without oxygen. Steam reforming of hydrocarbons proceeds according to the following main reaction (for methane):CH4+H2OCO+3H2  (1)
Similar reactions take place for other hydrocarbons. Normally the following reaction also proceeds on catalysts for steam reforming:CO+H2OCO2+H2  (2)
Steam reforming is highly endothermic and requires high temperatures typically above 800° C. in the reactor outlet to give acceptable conversions of the methane in the feed.
Main final products of Fischer-Tropsch synthesis are among others diesel and naphtha. The value of the naphtha is lower than the diesel. It is therefore known to recycle naphtha to the reforming section of the plant. WO-A-2013/033812 discloses a process (FIG. 5 herein) in which a hydrocarbon feedstock in the form of a natural gas stream after being desulfurized and pre-reformed is divided in two reforming process lines. One reforming process line passes through a steam methane reformer (SMR) and the other through an autothermal reformer (ATR). The thus reformed gases are combined into a single synthesis gas stream and then converted into diesel and naphtha via Fischer-Tropsch synthesis. Naphtha is recycled to the reforming section as well as part of the tail gas produced during the synthesis. This citation is silent about how much of the hydrogen and carbon monoxide of the combined synthesis gas is produced by SMR. In addition, the recycle of naphtha may require a higher steam-to-carbon molar ratio to avoid carbon formation in the pre-reformer or steam reformer and/or soot formation in the ATR. Higher steam-to-carbon molar ratios increase the capital expenses of the plant as more water has to be carried in the process.
Similarly, WO-A-2006/117499 (in particular FIG. 3 herein) discloses also a process in which pre-reformed gas is split into two lines. One line is passed through an ATR and the other parallel line through an SMR. The reformed gases from both lines are combined and are used in a plurality of downstream processes such as methanol, ammonia and Fischer-Trospch synthesis. Tail gas from Fischer-Trospch synthesis is recycled to the ATR, but not to the SMR. It is stated that the tail gas recycle is adjusted to meet the requirements of the downstream processes. This citation is also silent about how much of the hydrogen and carbon monoxide of the combined synthesis gas is produced by SMR.
In a plant for producing methanol, the desired stoichiometry of the synthesis gas to be sent to the methanol section is often expressed by the so-called module, M, where M=(X(H2)−X(CO2))/(X(CO)+X(CO2)); X is the mole fraction of the respective component. An optimal value of M is often stated to be 2 or slightly above, i.e. 2.0-2.10.
A stand-alone ATR unit (optionally with an upstream adiabatic pre-reformer) produces a synthesis gas with an M-value typically in the range 1.8.1-9 depending upon the operating conditions and the feedstock composition. In order to obtain a better module various methods may be used. These include recycle or import of hydrogen and removal of carbon dioxide. It has also been described in the patent literature, e.g. WO-A-2013/013895 to include a parallel SMR line to meet the desired value of M of about 2. An SMR produces a gas with an M value higher than 2. Hence, it is known to adjust the ratio of the synthesis gas produced by the ATR line and the SMR line to meet the desired value of M of about 2 in the final synthesis gas mixed from the two lines to be sent to a methanol unit. A similar citation is US-A-2007/004809. Again, this citation is silent about how much of the hydrogen and carbon monoxide of the combined synthesis gas is produced by SMR.
Conventional plants currently in operation for production of diesel via Fischer-Tropsch synthesis comprise a single line in which pre-reformed gas is passed through an ATR with Fischer-Tropsch tail gas and an oxygen containing gas. The amount of tail gas is adjusted to produce a synthesis gas with the required H2/CO molar ratio which is typically about 2. When this method of controlling the ratio between H2 and CO is used, a significant part of the total amount of tail gas may not be recycled, because this would produce a synthesis gas with too low ratio of H2 to CO. The part of tail gas which is not recycled may be used as fuel for process heaters and other purposes in the process. In case excess tail gas is available beyond these purposes, this represents a loss of overall efficiency.
It is well known that using a SMR instead of the ATR in a Fischer-Tropsch process results in a lower overall plant efficiency. This is i.a. due to external heat requirements for the SMR and the fact that ATR produces a synthesis gas more suitable for a Fischer-Tropsch synthesis stage than SMR. For example, for Fischer-Tropsch units it is advantageous to conduct Fischer-Tropsch-synthesis with a low inert concentration. Specifically, for low temperature units with cobalt based catalyst, all components except carbon monoxide and hydrogen may be considered inert. SMR produces a gas with a significantly higher inert content than ATR. Hence, in FT-plants the desired synthesis gas stoichiometry not only needs to have a H2/CO-molar ratio of about 2 but also a low inert level. Inerts include for example nitrogen, argon, methane, and often also carbon dioxide.
It is also well known that a non-catalytic partial oxidation unit (PDX) may operate in parallel with an SMR. This is however expected because the PDX unit produces a gas with H2/CO molar ratio below 2, while the SMR produces a gas with H2/CO molar ratio of well above 2 and in most cases higher than 3. Hence combining here the gases in order to obtain a synthesis gas with the desired H2/CO molar ratio of 2 for the purpose of downstream Fischer-Tropsch is straightforward.
It is an object of the present invention to provide a process for production of synthesis gas in a plant for production of diesel or other synthetic hydrocarbons with increased plant efficiency.
It is also an object of the present invention to provide a process for production of synthesis gas in a plant for production of diesel or other synthetic hydrocarbons with increased plant efficiency while at the same time being able to operate at low steam-to-carbon molar ratios in the ATR or pre-reforming stages.
These and other objects are solved by the present invention as recited in the appended claims.